Reid famously defines an efficient cause to be a being that had the power and the will to produce the effect. In his letter to James Gregory (14th June, 1785) Reid understands a law of nature to be “a purpose or resolution of the author of nature, to act according to a certain rule”. Later, in his Essays on the Active Powers of Man (1788), Reid places the emphasis on the laws being the rules themselves. A law of nature is now “a rule according to which the efficient cause acts” and “a thing conceived in the mind of a rational being, not a thing that has real existence” (Essay IV, IX, 344).
Reid states that laws of nature are either physical or moral. Physical laws of nature “neither restrain the power of the Author of nature, nor bring him under any obligation to do nothing beyond their sphere” (Essay IV, IX, 345). According to Reid, God has sometimes acted contrary to the physical laws, and perhaps sometimes acts without regard to them. Moral laws of nature are the rules proscribed to rational creatures by God. Although they ought to be obeyed, they are often transgressed by humans.
If a law of nature is a rule that is conceived of in the mind of God, and if God is under no obligation with respect to physical laws, then is God similarly under no obligation with respect to moral laws? This question is important because Reid’s first argument in support of the existence of moral liberty relies on the premise that God would not deceive us. Reid claims that we have a natural conviction that we act freely and that to suppose that this sense is fallacious is to impute a lie to God and hence to lay a “foundation for universal scepticism” (Essay IV, VI, 312). What grounds this confidence of Reid? Is there in the background some metaphysical argument purporting to show that God cannot lie, or is the claim merely that God would not lie, based on the evidence of what we know of God’s nature through general revelation in nature or special revelation in scripture?